Acceleration of Galactic Supershells by Lyman Alpha Radiation

TL;DR

This paper demonstrates that Lyman Alpha radiation pressure can accelerate galactic supershells to high velocities, potentially enabling them to escape galaxies and enrich the intergalactic medium, with properties consistent with observed high-redshift galaxies.

Contribution

It provides the first detailed calculation showing Lyman Alpha radiation pressure alone can drive supershells to escape velocities, linking radiation physics to galaxy evolution.

Findings

Lya radiation pressure can accelerate supershells to 200-400 km/s.

Predicted supershell properties match observed high-redshift galaxy features.

Mass outflow rates can reach 100% of star formation rates.

Abstract

Scattering of Lyman Alpha (hereafter Lya) photons by neutral hydrogen gas in a single outflowing 'supershell' around star forming regions often explains the shape and offset of the observed Lya emission line from galaxies. We compute the radiation pressure that is exerted by this scattered Lya radiation on the outflowing material. We show that for reasonable physical parameters, Lya radiation pressure alone can accelerate supershells to velocities in the range v_sh=200-400 km/s. These supershells possibly escape from the gravitational potential well of their host galaxies and contribute to the enrichment of the intergalactic medium. We compute the physical properties of expanding supershells that are likely to be present in a sample of known high-redshift (z=2.7-5.0) galaxies, under the assumption that they are driven predominantly by Lya radiation pressure. We predict ranges of radii r_sh=0.1-10 kpc, ages t_sh=1-100 Myr, and energies E_sh=1e53-1e55 ergs, which are in reasonable agreement with the properties of local galactic supershells. Furthermore, we find that the radius, r_sh, of a Lya-driven supershell of constant mass depends uniquely on the intrinsic Lya luminosity of the galaxy, L_alpha, the HI column density of the supershell, N_HI, and the shell speed, v_sh, through the scaling relation r_sh ~ L_alpha/(N_HI v_sh^2). We derive mass outflow rates in supershells that reach ~10-100% of the star formation rates of their host galaxies.